(a) Derive an SM chart for the elevator controller in Problem 16.29.

(b) Realize the SM chart in (a) using a PLA and D flip-flops.

Problem 16.29

The block diagram for an elevator controller for a two-floor elevator follows. The inputs FB₁ and FB₂ are 1 when someone in the elevator presses the first or second floor buttons, respectively. The inputs CALL_1 and CALL₂ are 1 when someone on the first or second floor presses the elevator call button. The inputs FS₁ and FS₂ are 1 when the elevator is at the first or second floor landing. The output UP turns on the motor to raise the elevator car; DOWN turns on the motor to lower the elevator. If neither UP nor DOWN is 1, then the elevator will not move. R₁ and R₂ reset the latches (described below); and when DO goes to 1, the elevator door opens. After the door opens and remains open for a reasonable length of time (as determined by the door controller mechanism), the door controller mechanism closes the door and sets DC = 1. Assume that all input signals are properly synchronized with the system clock.

(a) If we were to realize a control circuit that responded to all of the inputs FB₁, FB₂, CALL₁, CALL₂, FS₁, FS₂, and DC, we would need to implement logic equations with nine or more variables (seven inputs plus at least two state variables). However, if we combine the signals FB_i and CALL_i into a signal N_i(i = 1or2) that indicates that the elevator is needed on the specified floor, we can reduce the number of inputs into the control circuit. In addition, if the

signal Ni is stored so that a single pulse on FB_i or CALL_i will set N_i to 1 until the control circuit clears it, then the control circuit will be simplified further. Using a D flip-flop and a minimum number of added gates, design a storage circuit that will have an output 1 when either input (FB_iorCALL_i ) becomes 1 and will stay 1 until reset with a signal R_i

(b) Using the signals N₁ and N₂ that indicate that the elevator is needed on the first or second floor (to deliver a passenger or pick one up or both), derive a state graph for the elevator controller. (Only four states are needed.)

(c) Realize the storage circuits for N₁ and N₂ and the state graph.

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Status NEW Posted 08 May 2017 09:05 AM My Price 18.00

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